1. Field of the Invention
The present invention relates to a process and apparatus for forming relatively small diameter holes in relatively thin metallic plates by means of a pulsed laser beam.
2. Brief Description of the Prior Art
Processes and apparatus for drilling holes in thin sheet metal pieces or laminated plates by using high energy laser beam pulses are well known in the art. French Pat. No. 2,511,523 describes a process and apparatus for the verification and control of the position of the laser beam generator with respect to the work piece, but does not involve a system which forms each of the individual holes to a predetermined configuration. This patent involves the use of a light sensor to locate the laser beam with respect to the work piece but does not compensate for the variations in thickness of the work piece to ensure that the holes will have the desired configuration.
European Pat. No. 0013657 also descibes a method and apparatus for the use of a laser beam to drill holes in a work piece. In the system described therein, the radiation passing through the drilled hole is detected and utilized to control the laser beam generator. When a backing means is utilized with the work piece, the backer transmits impinged radiation from the hole to a detector coupled with the laser beam control so as to terminate the laser beam.
It is known that a pulse of a high energy beam, such as that delivered by a laser, may drill or form a hole in a relatively thin metallic work piece. For increasing thicknesses of the material and/or larger diameters of the holes to be drilled, it is, quite obviously, possible to increase the energy of the beam pulse (either by increasing the energy of the beam, increasing the duration of the pulse, or increasing both simultaneously) and to vary the focal distance of the laser beam. However, for reasons well known to those skilled in the art, the holes drilled in this manner exhibit irregularities when the thickness of the material to be drilled exceeds a certain threshold, except when the diameters of the holes are extremely small (for example on the order of 0.1 milimeter or less). Under these circumstances, the hole is the form of a more or less "crater" which normally renders it unsuitable for the intended application, which may be the production of elements with multiple perforations for sudation cooling, or for acoustic or antivibration panneling.
For this reason, it is generally preferable for relatively thick elements and/or holes with relatively large diamters (thicknesses on the order of 0.8 milimeters and diameters of several tenths) to form them with a succession of pulses. It is obviously necessary to control the succession of pulses directly or indirectly, as any drifting of one of the characteristics of the pulses may affect the quality of the finished hole. Certain deviations of the laser beam cause a reduction, or a change in distribution of the energy of the succession of pulses may result in a "blind" hole or one with a poorly defined opening. The same factors are true in cases where the thickness of the work piece varies in the area to be drilled, which occurs frequently when the work piece is a laminated element.